Photoplethysmography (PPG) is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue and estimate cardiovascular and respiratory parameters. A PPG sensor system typically includes a light source and a light detector, and infrared (IR) light emitting diodes (LEDs) are usually used as the light emitting component. PPG is often used to make non-invasive measurements at the skin surface. The accuracy of extracting cardiovascular- and respiratory-related information from the PPG signals, however, is usually not satisfactory due the PPG signal being generated from a single-wavelength light that can only provide the overall blood volume changes along the light penetration path, but not the blood volume changes of a specific layer. For example, IR PPG measures the sum total of volume changes in any and all blood vessels (e.g., large and small arteries, arterioles, capillaries, venules, and veins) throughout the skin, while the blood pulsation signal of a specific layer like pure arterial blood volume changes cannot be separated out from the IR PPG signal. The inability to distinguish depth of single-wavelength PPG measurements intrinsically degrades the performance for estimating cardiovascular and respiratory parameters, which requires the physiological information of the blood vessels in certain depths.
Pulse Transit Time (PTT) is the time it takes the pulse pressure waveform to propagate through a length of the arterial tree, which is a promising method to measure blood pressure (BP) in a continuous and cuff-less manner. In PTT measurement, PPG is commonly used to mark the arrival of the pulse wave at a peripheral site. Because arteries and capillaries are different in blood vessel wall components and blood circulation paths, arterial blood volume waveform and capillary blood volume waveform have different morphologies, as well as a phase shift. In this sense, IR PPG or any other single-wavelength PPG signal is a superposition of various pulse wave functions of blood vessels in different types and depths, thus unable to reflect the pure arterial blood change in the deep layer of the skin. Therefore, PTT cannot be precisely measured with a single wavelength PPG in which the capillary blood pulsations fundamentally weaken PTT's BP tracking ability.